A document entitled "Assessment of optical communications systems for data relay satellite" by J. L. Perbos and B. Laurent published in IAF 86 relates to the possibility of using optical technologies for providing the high data rate (HDR) communications links of the future European data transmission satellite system. Two particular optical technologies are envisaged in this document for providing inter-satellite communications: one is based on using CO.sub.2 lasers operating at 10.6 .mu.m, and the other on semiconductor laser diodes operating in the band 0.8 .mu.m to 1.3 .mu.m. In the context of inter-satellite optical links, one such document describes direct modulation of semiconductor lasers which requires the microwave signal reaching the first satellite to be demodulated prior to establishing a direct optical link between the two satellites, and which then requires the information regenerated on board the second satellite to be remodulated for retransmission to the ground at microwave frequencies.
Thus, when using a radio link (e.g. microwave frequency - optical frequency) it is not possible to retransmit without passing via baseband, i.e. without demodulation and regeneration.
The object of the invention is to mitigate this drawback by providing transitions between microwave frequencies and optical frequencies which are as simple as possible.